Image devices convert optical images into digital data. The image devices generate digital data of the optical images by converting the optical images into analog electrical signals and then by converting the analog electrical signals into digital electrical signals. Image devices are used, for example, in digital cameras to sense optical images and to convert the optical images into digital data.
The image devices include an image sensor and a color filter. The image sensor converts optical images into electrical signals. The color filter filters light by colors before the light is incident on the image sensor. The image sensor senses a luminance of the incident light. The image sensor includes photodiodes for generating electrical signals based on the luminance of the incident light. The image sensor including the photodiodes may be, for example, a complementary metal oxide semiconductor (CMOS) image sensor or a charged coupled device (CCD) image sensor.
Each pixel includes a photodiode. The image sensor includes a plurality of photodiodes. Hereinafter, a pixel is referred to as red, green, blue or cyan pixel based on the color represented by the pixel. Red, green, blue and cyan pixels include red, green, blue and cyan color filter elements, respectively. An array of the red, green and blue pixels are disposed in a mosaic form, e.g., a Bayer pattern disclosed in, for example, U.S. Pat. No. 3,971,065.
FIG. 1 is a view illustrating a conventional color filter having red, green and blue pixels arranged in the Bayer pattern. Photodiodes of an image sensor are disposed below each of the R, G, and B color filter elements to sense the luminance of light incident on the photodiodes that are filtered by the color filter elements.
Each of the photodiodes generates analog electrical signals based on the luminance of the sensed light, and the analog electrical signals are color-processed to obtain digital data of an image. The analog electrical signals obtained by the photodiodes are converted into digital electrical signals via an analog-to-digital converter (ADC), and the digital electrical signals are color-processed to obtain digital data of the image.
Output signal values of the photodiodes that sense light from the B color filter elements are much less than output signal values of the photodiodes that sense light from the R or G color filter elements when using the color filter having the conventional Bayer pattern. That is, when measuring output signal values of the photodiodes that sense light from the B, R or G color filter element while increasing luminance of a white light source, sensitivity of photodiodes to light from the B color element is smaller than the sensitivity of photodiodes to light from the R or G color element.
FIG. 2 is a graph illustrating the output signals of the photodiodes of R, G and B pixels. Referring to FIG. 2, when the output signals of the photodiodes of each the R, G and B pixels are measured while increasing the luminance of the white light source, the sensitivity of photodiodes to light from the B color filter is about half of that of the R or G color filter. The output signal values of the R or G pixel stop increasing at a luminance at which the output signal values of the R or G pixel are saturated. The output signals at or beyond such luminance level cannot be used as output signals. Therefore, the maximum output signal of the B pixel is about half of that of the R or G pixel.
Each of the R, G, and B pixels can be designed to have the same saturation output to overcome the weak sensitivity of photodiodes to light from the B color filter. However, the sensitivity of photodiodes to light from the B color filter is about half of that of the R or G color filter as illustrated in FIG. 2. Therefore, there is a difference in the saturation output signal values that can be actually used as an output signal due to a difference in saturation luminance between the color elements. If a number of the saturation output signal that is converted into electrons is denoted as N, shot noise is denoted as √{square root over (N)}. The maximum signal-to-noise ratio (S/N) is 10 log N. That is, the maximum S/N depends directly on the number of electrons of the saturation output signals that are used as signals. Even if the output signals of the photodiodes that sense light from the B color filter is used after amplifying it by 2 times to equalize the sensitivity of photodiodes to light from the B color filter to the sensitivity of photodiodes to light from the R and G color filters, the same S/N can be maintained because the number of electrons of the output signals of the photodiodes that sense light from B color filter remains the same.
If a noise floor per pixel channel is stable and the sensitivity to light from the B color filter is ½ of that of R or G color filter, the saturation output of the photodiodes that sense light from the B color filter with respect to the white light; source is ½ of the saturation output of the photodiodes that sense light from the R and G color filters. Accordingly, a dynamic range limit is limited by B pixel. Since the dynamic range limit of the image sensor is decreased, the sensitivity to light from the B color filter needs to be increased to improve the S/N.